Wing end plates help to increase the effectiveness of the rear aerodynamic parts. Helping to make sure airflow is ducted to the rear wing and doesn't spill over the sides, while reducing drag. Rear wing end-plates effectively make the wing span operate as if the span was a lot bigger- while retaining a more compact design.
Without wind tunnel time and some serious calculations there is no way us amateurs can work out the optimum size/design of the end plates. However 75-100mm above the wing upstand and 150-200mm below will work pretty well. Increasing these dimensions may increase downforce but I doubt that you will notice. The bigger the end plates the greater the forces acting on them in corners so have to be appreciably stronger (and heavier).
How do you plan to attach them to the existing wing?